Please don't take me as an expert on the Goldline, but I know on the older models (if yours is a 2001 or 2002), that the salinity was based upon the cell condition, water temperature and actual salinity level. As you're suspecting that your cell is getting old (which I would agree with), your higher salt level (4000 + ppm) is actually allowing your cell to continue to generate chlorine. This is consistent with how an AutoPilot unit would operate with a failing cell too. Higher salt levels compensate for the reduced ability of the cell to conduct the amps and volts across the cell blades.
If you winterize, you may be able to stretch this cell to the end of the season, they you will need another one. My recommendation if this is what it happening, is to wait until you open your pool next season to replace the cell. If you get the cell now, you're going to loose 4-6 months of cell warranty because you won't be using it while your pool is winterized.

If you don't winterize, you can go about another month or two with the high salt levels and still maintain chlorine in your pool, but make sure the cell remains clean of the white calcium scale. Then you will eventually need to get a new cell.

Keep in mind that when you do replace the cell, the unit should recognize the high salinity level so you will have to dilute your pool to get your salt level back down. Otherwise the unit will indicate a HIGH SALT level warning.

Hope this helps,